FIG. 17 is a cross section of a prior art electroplating apparatus. In the figure, reference numeral 1 designates a cup in which a substrate 2 is processed, numeral la designates a plating solution inlet, numeral 1b designates a plating solution outlet, numeral 3 designates a power supply, numeral 4 designates an anode, numeral 5 designates a cathode contact pin contacting and holding the substrate 2, numeral 6 designates a plating bath comprising a resin, numeral 6a designates a drain of the plating bath 6, numeral 7 designates a plating solution storage tank, numeral 7a designates a heater for heating the plating solution in the storage tank 7, numeral 8 designates a pump for circulating the plating solution, numeral 9a designates the surface of the plating solution, numeral 11 designates oxygen in air, and numeral 12 designates Au particles. Arrows 9 show the flow of the plating solution. A dotted line 10 shows a leakage current flowing through the plating solution.
A description is given of the function and the operation.
The substrate 2 is disposed in the cup 1 so that the front surface on which a metal layer is to be formed by electroplating looks downward, and tips of at least three cathode contact pins 5 which are fixed to the cup 1 are in contact with the front surface of the substrate 2, whereby a negative electrode of the power supply 3 is electrically connected to the front surface of the substrate 2. The solution inlet 1a of the cup 1, the solution circulating pump 8, the storage tank 7, and the plating bath 6 are connected with pipes. The circulation of the solution is indicated by the arrows 9. The plating solution is injected into the cup 1 from the inlet 1a. When the surface 9a of the plating-solution reaches the front surface of the substrate 2, the solution flows from the plating bath 6 through the solution outlets 1b on the side walls of the cup 1. Then, the solution in the plating bath 6 is drawn through the drain 6a into the storage tank 7. The solution in the storage tank 7 is again sent to the cup 1 by the solution circulating pump 8. The anode 4 is disposed in the cup 1 opposite to and parallel with the surface of the substrate 2. When the cup is filled with the plating solution and a current is applied across the anode 4 and the substrate 2 which is connected to the cathode contact pins 5, electrolysis of the plating solution occurs, and a plating metal is deposited on the front surface of the substrate 2. In this way, the electroplating is carried out.
In the above-described prior art electroplating apparatus, however, since the substrate 2 is disposed with the front surface to be plated with metal looking downward, bubbles caused by the plating solution circulating system or bubbles of gas caused by the cathode reaction are unfavorably attached to the front surface of the substrate, resulting in an imperfect plating. Further, since the front surface of the substrate looks downward, the substrate must be manually set by an operator using tweezers, and it is difficult to automate the placement of the substrate.
The inventor of the present invention proposed an electroplating apparatus in which a substrate is set in a cup with its front surface looking upward, in Japanese Published Patent Application No. Hei. 1-294888. In this apparatus, however, a complicated mechanism is required for transferring the substrate from a substrate stage to a vacuum chuck of a robot, and an automatic transfer of the substrate using a simple uniaxial robot which moves only in vertical and horizontal directions is impossible.
Further, in the prior art electroplating apparatus of FIG. 17, although an electric circuit comprising the power supply 3, the anode 4, and the cathode 5 is provided for each cup, actually leakage current is generated with the electrolytic plating solution as a medium as shown in FIG. 17. In order to avoid the leakage current, it is necessary to provide a plating bath for each cup. If more than two cups are disposed in the same plating bath, not only anodes but also cathodes of the respective cups are at the same voltage, i.e., the respective cups are equivalent to a parallel circuit. Therefore, in a case where a constant current supply employed as the power supply 3, if the contact resistance between the cathode contact part and the substrate varies from cup to cup, different amounts of current are applied to the respective substrates, so that the thicknesses of the plated layers on the respective substrates in the same plating bath cannot be precisely controlled. Further, in the prior art electroplating apparatus, simultaneous processing using two or more current conditions is impossible due to the interaction caused by the leakage current in the plating bath.
In order to solve the above-described problems, the inventor of the present invention proposed an improved electroplating apparatus in which a substrate 2 is disposed in each cup 1 with its front surface looking upward and the respective cups 1 include anodes 4 of the same voltage, in Japanese Published Utility Model Application No. Hei. 3-85470. In this structure, however, not only the anode but also the cathode must be independent to solve the above-described problems.
That is, the above-described problems are easily solved if a plating solution circulating system is provided for each cup, i.e., each wafer. However, when a lot of substrates are processed at the same time in an electroplating apparatus, the concentration of the plating solution must be uniform throughout the cups containing the respective substrates, so that the plating solution circulating system for each cup must have means for controlling the concentration of the solution. Therefore, this structure is of no practical use.
Furthermore, since the substrate is in contact with the tips of the cathode contact pins fixed to the cup, even a slight difference in the heights of the cathode contact pins affect the contact between the substrate and the plating solution, and a variation in the flow velocity of the plating solution causes an uneven thickness of the plated layer on the front surface of the substrate. Therefore, the height of the cathode contact pin must be precisely adjusted. However, such a precise adjustment is difficult and requires much time.
Further, when a plating solution which is relatively unstable and easily produces abnormal Au deposition when it is decomposed, for example, Au sulfite, is employed, the cup must be hermetically sealed so that the plating solution is not in contact with air. In the prior art plating bath 6 in which the plating solution is exposed to air, sulfite ions are oxidized due to the mixing of oxygen 11 in air and the plating solution, and Au particles 12 which cause the decomposition of the plating solution are produced. In this case, the lifetime of the plating solution is reduced compared to plating in a beaker with less agitation and invasion of air.
Further, at the surface of the plating solution, recrystallization of Au occurs due to evaporation of the solution, resulting in electroless deposition of Au.
Further, as illustrated in FIG. 10, when Au is selectively plated in a via-hole 2a in a substrate 2 and having a width of several tens of microns and a depth of several tens to several hundreds of microns as well as on part of the substrate 2 in the vicinity of the via-hole 2a using a photoresist mask 2b, since the ratio of the internal area of the via-hole 2a to the whole area to be plated is large, the plating rate is adversely affected by the variations in the depth of the via-hole during plating.